1. Field of the Invention
This invention relates to the production of electricity and steam from steam-heating power plants and, more particularly, to apparatus for control of such power plants to meet changes in electrical and steam-heat demand placed on a power facility.
2. Description of the Prior Art
Conventional manufacturing facilities generally require use of a wide variety of energy in the form of electricity and/or steam to operate equipment essential to the manufacturing of the desired product, as well as for heating and comfort control. The electrical requirements of a given facility can of course be met by purchasing electricity from utility companies. To a large degree, these utilities produce electricity by the well known Rankine Cycle employing water as the Rankine Fluid by way of combustion of fossil fuels. Electricity is also produced by hydroelectric or nuclear methods. However, in the fossil fuel systems steam, produced at a high pressure and temperature in boilers which burn fuel to heat water passed therethrough, is expanded nearly adiabatically through succeeding stages of axially rotating turbine blades. At each stage of expansion the energy content of enthalpy of the steam is lowered, raising the mechanical rotational energy transmitted to a rotating shaft attached to the blades by an amount corresponding to the decrease in enthalpy less, of course, the inevitable losses due to friction, etc. The energy transferred to the rotating shaft is converted into electricity by means of a generator coupled to the rotating shaft.
A limit, however, exists on the number of expansion stages possible in a turbine and on the amount of energy which can be eventually extracted as electrical power from that energy originally imparted to the steam from the burning of the fuel. The steam vapors exhausted from the last turbine stage of expansion still contain the latent heat of vaporization and are usually passed to an apparatus wherein the steam is suitably treated to cool and condense the vapors, with the heat thereby extracted from the steam being taken to waste. While some condensation does occur in the expansion process due to the thermodynamic properties of steam, the greater percentage of the energy originally delivered to the steam in the boilers can be practically considered as lost. The most efficient power generating plants which produce electricity from fossil fuels convert to electricity only 46 percent of the energy originally imparted to the steam, with a remaining 54 percent of the energy originally imparted to the steam being wasted.
While the steam vapors which exit the last turbine stage of the utility company may be sold as a source of heat, industrial facilities or others requiring the steam in manufacturing or in heating may not be located convenient to the utility plant. In these cases, the transmission of steam over long distances to the end user would be uneconomical and would result in a substantial transmission heat loss. Thus, to take advantage of the heat remaining in steam following exhausted from the last turbine stage of an electrical generating apparatus system, industrial designers have recognized that local generation of the electricity within an industrial facility would enable efficient use of the energy of the exhausted steam which would otherwise be wasted to the environment. Therefore, manufacturing industries using large quantities of steam can find it very economical to generate steam at a high pressure, pass the steam through a turbine to generate electricity used within the plant, and then employ the exhausted steam at a lower pressure and temperature for use in manufacturing processes. Such systems are herein referred to as "steam-heating power plants." Since the electrical generating apparatus in such a facility would not need to take the steam to as close to the low temperature condensing conditions (e.g., from about 75.degree. to 130.degree. F.) as would be necessitated by utility companies in which the steam would otherwise be discharged to waste, work in the form of electricity can be extracted from the steam passed through the turbine to about a 90 percent efficiency. Turbines selected to operate in ranges where conditions of temperature at exhaust or extraction can be gainfully used for other purposes are often called "topping turbines," with the electrical energy being produced being called "topping power." In contrast, turbines employed in utility companies which, for economic reasons must take the steam to as close to low temperature condensing conditions as possible, are termed "condensing turbines" with electricity being produced being termed "condensing power."
Steam-heating power plants typically include at least one topping turbine and at least one condensing turbine, and are generally supplied with steam from at least one steam generating source, e.g. a fossil fuel-burning boiler. High-pressure steam, e.g., 400 to 1500 psig, is generally first passed from the steam source through the topping turbines for generation of electricity. Lower pressure steam exhausted from the topping turbines is herein termed "intermediate pressure steam" and possesses a pressure between that of the high pressure steam fed to the turbine and that pressure corresponding to condensing conditions. The intermediate pressure steam will typically have a pressure of from 100 to 550 psig, although this may vary widely depending on equipment design and other factors. This intermediate pressure steam is in part passed to the condensing turbines for generation of additional electricity. Steam required for meeting heating and other process requirements may be withdrawn from the topping turbines or condensing turbines at any stage thereof, or as is more typical such steam may be withdrawn from the line containing the intermediate pressure steam which is exhausted from the topping turbines. The rotational mechanical energy from the topping and condensing turbines is generally transferred to separate generators for production of electricity. The electrical outputs from these generators are usually to a common electrical network or load.
In designing an industrial complex to include a steam-heating power plant, an ideal energy balance is provided when the demand for electrical energy can be supplied by electricity produced in the topping turbines with demand for heat energy in the form of steam being equivalent to that steam exhausting from the topping turbines.
In practice, however, an ideal balance is seldom achieved. Many factors unpredictable and beyond ordinary human control cause disruptions in energy consumption patterns of the individual processing sections which form a part of the steam-heating power plant. Examples of these disturbances include changes in weather conditions, equipment down-time, intermediate storage, market conditions, etc. If there then is a greater demand for steam than for electricity, and if steam demands are to be met by the steam-heating power plant, the opportunity is lost to produce cheap electricity at this facility since that portion of the steam diverted to the heating requirements cannot be used to generate electricity. If there is a greater demand for electricity than can be generated from the steam produced in the topping turbine system, then the electricity must also be supplemented. This supplemental electricity can be obtained from an alternative source, such as by purchasing electricity from a utility, or can be produced by passing additional amounts of steam to a condensing turbine or even by an internal combustion engine. However, whatever the source of the supplemental electricity, it is always more expensive than that which can be generated by the topping turbines.
Prior art practitioners have attempted to control the generation of electricity and steam by such a system by sensing the initial steam boiler pressure and regulating, as with a suitable responsive control mechanism, the fuel fed to the boilers to keep the boiler pressure constant. The pressure of the intermediate pressure steam exhausted from the topping turbine is also sensed and the quantity of steam permitted to pass through the topping turbines is, as by a suitable responsive control mechanism, regulated to maintain a constant pressure in the steam line containing the intermediate pressure steam. Generation of electric power is controlled by sensing the frequency of the power that is generated and regulating the steam passed through the condensating turbine employing a suitable responsive control mechanism to maintain constant frequency of the generated electricity.
However, this method of control has proved unsuitable in situations where there are fluctuations in steam or electrical demand as is generally the case in any manufacturing facility. Interaction between the various pressure and frequency sensing mechanisms causes cycling between the control mechanisms which are responsive to their various signals.
For example, a decrease in the electrical demand results in an increased electrical frequency of the power plant. This is detected by the frequency control mechanism which then reduces the quantity of steam permitted to pass through the condensing turbine in an attempt to maintain a constant frequency. However, this has the effect of raising the steam pressure in the topping turbine exhaust line which feeds the condensing turbine and from which steam is withdrawn to the manufacturing plant. This pressure change is detected by the pressure sensor on this intermediate pressure steam line, causing a corresponding reduction in the quantity of steam permitted to pass through the topping turbines so as to maintain a constant steam pressure. However, this results in a decreased generation of topping power and has a delayed effect of lowering the frequency detected by the frequency sensor. The frequency controller would then seek to increase the quantity of steam passed through the condensing turbine to offset the detected frequency decrease, thereby causing more of the steam exiting the topping turbine to be consumed in the condensing turbine and resulting in a pressure drop in the topping turbine exhaust line. This pressure drop is then in turn sensed by the control mechanism on this line which increases the amount of steam fed to the topping turbines, resulting in a rise in the generated frequency. Once the cycle begins it continues indefinitely.
The prior art has attempted to overcome this problem of cycling by interposing a regulating valve between the steam line from the boiler which feeds the topping turbine and the intermediate pressure line receiving the topping turbine exhaust. By use of this regulating valve, commonly used in conjunction with a de-superheater, steam is expanded isenthalpically into the intermediate pressure pipe which receives the topping turbine exhaust. As before, the pressure of the topping turbine steam fed is sensed and operates to control the quantity of fuel fed to the boilers. A change in the electrical demand, as manifested by change in the generating frequency, would be sensed and caused to regulate the quantity of steam passed through the condensing turbine for increased or decreased generation of electricity therein. However, the controller which receives the pressure signal from the sensor on the intermediate pressure steam line, instead of regulating the quantity of steam passed through one or more topping turbine, instead regulates the valve which by-passes the topping turbines, allowing steam produced in the boiler to feed directly into the line which receives the topping exhaust.
While incremental changes in the pressure of the topping exhaust do not materially interfere with the frequency control mechanism operating to sense the frequency of the electricity generated in the topping turbine downstream of this exhaust, and while the use of such a by-pass valve reduces the cycling problem, it is readiy apparant that by-passing the topping turbines in this manner is disadvantageous since the opportunity is lost to generate the cheaper topping power. This decreased quantity of electricity must therefore be supplied instead from more expensive sources as discussed previously. This is a serious deficiency in an industrial steam-heating power plant due to the large quantities of steam that are employed.